1. Field of the Invention
This disclosure relates to the field of systems, devices and methods utilized to measure the level of mottle in multilayer laminates.
2. Description of Related Art
Multilayer laminates are generally panels comprised of two sheets of a substrate (such as, but not limited to, glass, polyester, polyacrylate, or polycarbonate) with two or more polymer interlayers sandwiched therebetween. Contemplated polymer interlayers include, but are not limited to, polyvinyl butyral (PVB), polyurethane (PU), poly(ethylene-co-vinyl acetate) (EVA), poly(vinyl)acetal (PVA), polyvinylchloride (PVC), polyethylenes, polyolefins, ethylene acrylate ester copolymers, poly(ethylene-co-butyl acrylate), silicone elastomers, epoxy resins and any acid copolymers and ionomers derived from any of the foregoing possible thermoplastic resins. Multilayer laminates can include multiple layer glass panels and multilayer polymer films. In certain embodiments, the multilayer polymer films in the multilayer laminates may be laminated together. In certain embodiments, these films may have coatings, such as metal, silicone or other applicable coatings known to those of ordinary skill in the art. The individual polymer films which comprise the multilayer polymer films may be laminated together using an adhesive as known to those of ordinary skill in the art.
Generally, multilayer laminates can be produced by the following method. First, one or more polymer interlayers are placed between two substrates to create an assembly. In certain embodiments, the layers could include a multilayer interlayer produced through a coextrusion process. Then, air is removed from the assembly by an applicable process or method known to one of skill in the art; e.g., through nip rollers, a vacuum bag or another deairing mechanism. Following the removal of the air from the assembly, the constituent parts of the assembly are bonded by a lamination process known to one of ordinary skill in the art such as, but not limited to, autoclaving. Amongst other applications, the resultant multilayer laminate glass panels from this process are utilized in architectural window applications and in the windows of motor vehicles and airplanes.
One of the problems in the manufacture of multilayer laminate glass panels is the presence of mottle in the final unitary structure. The term “mottle” refers to an objectionable visual defect in the final unitary structure, namely the appearance of uneven spots. Stated differently, mottle is a measure of the graininess or texture of the surface area of the inner polymer interlayer or polymer interlayers. It is a form of optical distortion.
It is believed that mottle is caused by small scale surface variations of the interfaces between layers of the laminate having different refractive indices. The refractive index of a layer is the measure of the speed of light through that substance. Mottle is theoretically possible with any multiple layer interlayer provided that there is a sufficiently large difference in the refractive index between the layers and there is some degree of interfacial variation.
The presence of mottle in the final unitary structure of a multilayer laminate glass panel can be problematic because a certain degree of optical quality is necessary in many (if not most) of the end-use commercial applications of multilayer laminate glass panels (e.g., vehicular, aeronautical and architectural applications). Thus, both the creation of multilayer laminate glass panels with commercially acceptable levels of mottle and a uniform and systematic method or device that can consistently measure the degree of mottle in a given multilayer laminate glass panel is paramount in the art of multiple layer glass panel manufacturing.
Traditionally, assessments of the degree or amount of mottle in a multilayer glass panel were determined by an inherently subjective process. In this process, mottle was visualized using a shadowgraph-based technique. A shadowgraph is an optical method that reveals non-uniformities in transparent media like air, water, or glass. In principle, the unaided human eye cannot directly see differences or disturbances in transparent air, water or glass. However, all these disturbances/differences refract light rays and, accordingly, they can cast shadows. A shadowgraph exploits this property of the ability of the disturbances or differences in a laminate to cast shadows and utilizes it to project an image of the non-uniformities in the laminate onto a screen.
A shadowgraph is created as follows. A source of light is utilized to provide a uniformly diverging light source. One prerequisite of the light source is that it be a point of light source. If the light source is not a point of light source then inhomogeneities in the light source which could affect the shadowgraph will be projected. In a first step, the multilayer laminate to be tested for mottle is placed somewhere in between the light source and a white background or screen. The light emitted from the light source passes through the multilayer test laminate and is then projected onto the white background or screen to produce the shadowgraph. A diagram illustrating how a shadowgraph is created is depicted in FIG. 1, which depicts a procedure for producing a shadowgraph with a multilayer laminate (specifically with a trilayer laminate).
Generally, as the uniformly diverging light source passes through a multilayer laminate, the direction of the light changes as it passes through layers of different refractive indices as described by Snell's Law. The direction of the light changes according to the ratio of refractive indices and the angle of the incoming light, relative to the plane of the interface. If the interface plane varies due to surface non-uniformities, the angle of the reflected light will vary accordingly. The non-uniformly refracted light leads to an interference pattern resulting in a projected shadowgraph image with light and dark spots.
In the traditional process for ascertaining mottle, the severity of the mottle was assessed and categorized by a side-to-side qualitative comparison of the shadowgraph projections for the multilayer test laminate with a set of standard laminate shadowgraphs representing a series or scale of mottle values ranging from 1 to 4, with 1 representing a standard of low mottle (i.e., a low number of disruptions) and 4 representing a standard of high mottle (i.e., a high number of disruptions), which is optically objectionable. For a better understanding of the traditional process, the pictures of the standard laminate shadowgraphs for the mottle standards 1, 2 and 3 are depicted in FIG. 2. Based upon a visual interpretation of which standard laminate shadowgraph picture the test shadowgraph projection best corresponds with, the test laminate is then placed into the mottle category of the corresponding standard laminate.
A problem in the currently utilized traditional mottle analyzer system is that this process (due to its subjective nature) is inherently susceptible to inconsistencies, person-to-person variation and human bias. Coupled with a recent demand for lower acceptable mottle levels from the commercial sector, this has created a need in the industry for a more quantitative and reproducible test to measure the degree or level of mottle for a multilayer laminate.